1. Field of the Invention
The present invention relates to a working fluid filtering device for an electrical discharge machine, in which fine metal sludge of a workpiece discharged during machining is filtered by a rolled filter media.
2. Description of the Related Art
To filter a working fluid in an electrical discharge machine, such as a wire-cut electrical discharge machine, filters have become widely used in which a filter media, such as synthetic fiber, cellulose, etc., is folded and packed into a cylindrical metal or plastic container. A filter, as stated herein, is a combination of a filter media and a filter vessel that contains the media.
As the filter continues to filter sludge discharged during machining, the sludge accumulates in the filter media. In a certain time, the sludge causes clogging and reduces the filtering capability. The filtering capability is a filter flow rate.
The filter flow rate of the clogged filter can be increased by increasing the pressure during filtration. Since the filter media and the filter vessel are limited in strength, however, the filtration pressure cannot be increased beyond the limited strength. In general, the timing for the replacement of the filter corresponds to the point in time when the strength limit is reached by the filtration pressure.
Japanese Patent Application Laid-Open No. 2-30433 discloses a technique in which a rolled filter media is used for filtration and a collection roller is driven to collect a used filter when the filter is clogged, thereby increasing the internal pressure of a filter tank. According to this technique, however, the filter media is interposed between the filter tank and a bottom board as it moves. Thus, there is a problem that contact parts of the filter media and a seal easily rub against each other and wear out, so that a working fluid is liable to leak from a sealed portion after continued use. Due to its possible wear, therefore, the filter media cannot be backwashed for repeated use. Since there is no provision for washing the filter, moreover, the collected filter cannot be reused.
Japanese Utility Model Application Laid-Open No. 62-022025 discloses a sub-filter technology intended to extend the life of a filter element in a main filtering device.
According to the prior art described above, the filter media is interposed between the filter tank and the bottom board as it moves. To prevent the contact parts of the filter media and the seal from rubbing against each other and wearing out, therefore, it is not advisable to increase pressure on the filter media. Thus, filter media with low filtration accuracy cannot be used in this field.